Tip on Removing Mill Scale

Tip on Removing Mill Scale

Mill scale...You know.....That hard black shit that you find on most steel you buy nowadays. It's on round tubing, square tubing, rectangular tubing, angle iron, H-beam, flat plate, etc. etc....I'm not sure if the mill scale is to protect the metal or this is happens during the manufacturing process?

Well, I've always ground, sanded, wire brushed, or sandblasted that hard mill scale shit to get down to the bare metal for painting. I've even tried burning the shit off without success.

Today I had some small plate steel items I needed to clean up (remove the mill scale). I spent over two hours on one piece with my el-cheapo sandblaster......not good...taking to long.....so I was just going to take them to an industrial sandblasting outfit on monday. For shits and giggles, I thought I would try something. I have a 1 gallon jug of pure muratic acid. I heard the muratic acid will clean up rust....maybe mill scale? I put one of the parts in a plastic container and splashed a bit of muratic acid on it......whew!!....the stink dam near dropped me to my knees. Opened the garage doors...aired things out and got rid of the stuff I had just poured in. I noticed on the piece, where the muratic acid had touched the mill scale, it was shinny bare metal underneath....Hmmmmm. So I tried something different. I put the item to be cleaned in a plastic container and put enough water (about a gallon) to just cover it... slowly added muratic acid (about a cup full) to the water. I didn't notice much of a smell this time? Anyway, I left the piece in the solution for a half hour...ran a soft paintbrush over the piece.....and wooo...hooo...the mill scale just brushed off so easily...right down to the bare grey metal underneath. I pulled the item out...blew off the excess water/muratic acid with compressed air...let dry a bit.....a quick light sandblast....just tickety-boo......"It's just that easy folks."

Just a safety note: Do not add water to muratic acid.............Add muratic acid to water only.

You really need to be careful with muratic acid. The fumes can kill you, and it will burn your skin. I would not be using an air compressor to blow it off. Instead rinse it off with water first. A better solution would be to use cold rolled steel instead of hot roll. Cold roll does not have the scale, and is stronger.
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Cliff you need to neutralize the acid. Try another tank of water with washing soda (lye) mixed in it.
When the parts come out of the acid, pat dry, dip into the lye bath (30 seconds), remove and rinse
with clear water. Dry and prep as usual. Muriatic acid is a bitch. It tends to stick to metal and continue
to etch even if you dry the piece thoroughly.

I've never run across this problem with any frames or tanks I've ever painted..Am I just missing it and painting over it..??I just looked at a Daytec frame sitting here and I don't see anything like that...I seem to remember hearing of mill scale.. but never had to deal with it..What am I missing..??

Lowrider. The tubing that the frames are made out of, are cold rolled steel. It does not have mill scale. Only hot rolled has. Some people refer to cold roll as "clean metal". One way to tell cold roll from hot roll is by looking at a factory edge. If the edge is nice and square like it was sawed it is cold rolled. If the edges are kind of rounded it is hot rolled.
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I understand that.. but why would anybody use hot rolled instead of cold rolled..?I understand that DOM and 4130 are considerably more expensive.. but is cold rolled much more...??I had always thought that hot rolled was a lesser quality of steel.... Is that true.. ??Doesn;t the tool that Alan linked to.. beat up the surface pretty well..I Painted a sprint car frame once where they used that or something similar to clean the welds and it left some decent nicks..

lowridertommy wrote:
I understand that.. but why would anybody use hot rolled instead of cold rolled..?I understand that DOM and 4130 are considerably more expensive.. but is cold rolled much more...??I had always thought that hot rolled was a lesser quality of steel.... Is that true.. ??Doesn;t the tool that Alan linked to.. beat up the surface pretty well..I Painted a sprint car frame once where they used that or something similar to clean the welds and it left some decent nicks..

The composition of the steel can be either hot rolled or cold rolled, it just depends on the finishing process used by the mill.1018 for example, comes either hot or cold rolled, either has the same mechanical properties..Hot rolled is a bit cheaper as it is not send through the mill again to do the cold rolling process, but then hot rolled will have mill scale on the outside.

One effect of using cold rolled is that any parts made with it will warp slightly.Cold rolled steel has internal stresses built up in it by the finishing process.Cutting shapes out of cold rolled bar stock releases some of the stresses and willslightly warp the part as the stress is released.

lowridertommy wrote:
I've never run across this problem with any frames or tanks I've ever painted..Am I just missing it and painting over it..??I just looked at a Daytec frame sitting here and I don't see anything like that...I seem to remember hearing of mill scale.. but never had to deal with it..What am I missing..??

from cycle ones site:

Each custom frame/hard tail section is shipped out with the mill scale still on. This raw state protects the motorcycle frame from rust as there are oils in the mill scale that protect the steel. It also allows the customer to finish the motorcycle frame by their choice of method and means. The most obvious reason is that what you see is what you get, all the frame welds are visible. Giving you the customer the ability to appreciate the craftsmanship. Not having to worry about what might be underneath the coating/grinding/sanding, as it is all visible!

Hmm.. I've never noticed that on their frames...Maybe, I just painted over it..lolol..I have everything sandblasted before mudd and paint.. so that would explain my never having to deal with it.. (but ya think I would have seen some of it.. )I've seen frames in the past with noticeable slag before though..